Electrochemical detectors are devices which measure selected gases, such as CO, CO.sub.2, and H.sub.2, CH.sub.4 and the like. These devices are particularly useful in measuring very small amounts of the selected gas such as for example, 2 to 10 parts per million, and must be accurate particularly were the threshhold limit value of a regulated substance is being measured.
Typically, electrochemical devices have a zero signal which is defined as a signal produced by the detector when the selected gas is not present in the cell and a span signal which is the signal produced by the detector when the selected gas is present. Numerous electrical circuits are available to convert span signals from the electrochemical cell into useful information either by a meter or recording device. Circuits for adjusting the zero and span signals are also known.
In addition to the conventional adjustment or calibration requirements, both the zero and span signals may be temperature dependent. Accordingly, it is necessary to compensate for any such temperature induced variation within the expected working range of the cell. One such compensating circuit is disclosed in U.S. Pat. No. 3,909,386. As described in that patent, the variations due to temperature in both span and zero signals are logarithmic functions rather than linear functions. The circuit described therein utilizes a thermistor designed to compensate for the span signal variations and a thermistor circuit designed to compensate for the zero signal variations. The two thermistors, however, do not mutually interact.
While it has been generally known that the changes in span and zero signals are not uniform, it has been found that the variation in zero current is normally much greater than that of the span signal. Moreover, it has been found that the variation due to temperature in zero current is normally greater than that which can be compensated with commercially available thermistors. Therefore, compensating circuits similar to that disclosed in U.S. Pat. No. 3,909,386 do not provide adequate compensation, particularly with respect to zero signal variations.
It is, therefore, an object of the present invention to provide a thermal compensating circuit which will adequately compensate for both variations in the span and zero signals. It is a further object of the present invention to provide a thermal compensation circuit which will adequately compensate the larger variation normally found to exist in zero signals of electrochemical detectors and to overcome the disadvantages inherent in those circuits utilizing thermistors that do not mutually interact.